Bill Tremayne and Trevor Kelly
The new loadings code, NZS 1170, will replace our current loadings code, NZS4203:1992, and will likely govern structural design and analysis in New Zealand until 2015 and beyond. We can expect that within this time our profession will adopt the technology available to us to evaluate the response of structures to earthquakes much more accurately than is done today. This will move us toward implementing performance based design by using nonlinear time history analysis. Current codes discourage time history analysis because the scaling of time history records results in larger earthquake response than alternate, linear elastic methods of analysis. The committee developing the draft loadings code is attempting to formulate the provisions so as to remove this impediment to more accurate analysis. This paper presents the results of a study to assess the impact of the proposed earthquake scaling factors on 8 prototype moment frame and shear wall buildings, 6 and 12 stories high, located on sites in Auckland and Wellington. Maximum response predicted by both the response spectrum and time history methods of analysis is compared. The need for different drift criteria, depending on the type of analysis used and the influence of near-fault effects, is highlighted by this study.
Paper P19: [Read]
Quincy Ma, John Butterworth and Barry Davidson
A technique is proposed for the purpose of analysing rocking structures using finite element methods. The proposed technique involves a minimum number of approximations and makes similar assumptions to those used in Housner’s rocking rigid block model. The example presented in this paper shows that the proposed method is an improvement on a number of previous analyses of rocking structures. Contact forces are consistent with contact conditions and no fictitious damping or other restoring forces are relied upon in the uplift stage. The effectiveness of the proposed method is evaluated by comparing displacement time histories obtained 1) by a finite element approach, 2) by solving Housner-type governing differential equations, 3) from published results of a previous study. Good matching of all three was achieved; however there are still opportunities for further refinement and improvement to the cumbersome parameter selection process.
Paper P20: [Read]
In New Zealand the racking strength of house bracing walls is currently determined from the BRANZ P21 test. House designers sum the racking strengths of all walls, using published data from sheathing manufacturers’ wall systems, to ensure that actual house earthquake strength exceeds the demand loading stipulated in NZS 3604. Continuity of construction is simulated in the P21 test by a partial end uplift restraint. A revised BRANZ test and evaluation method called EM3 uses a far greater end restraint and multiplies the measured racking strength by three factors (F1, F2 and F3) to give the evaluated seismic resistance. The paper explains the background to these factors and the additional provisions and recommendations. F1 was obtained by computer earthquake simulation to ensure houses would not deflect excessively under design level earthquakes. F2 is a systems effect factor based on racking tests on a full scale house and laboratory specimens. F3 is a factor used for specific construction and situations. The paper also details house torsional analysis computer simulations, and outlines the current BRANZ test program on half-scale house assemblies which is designed to quantify the F2 factor and confirm the torsional analysis findings.
Paper P21: [Read]
Ted Blaikie and Robert Davey
This paper outlines the development of a methodology that can be used to predict the seismic stability of a cracked, face-loaded unreinforced masonry (URM) wall. The methodology makes use of both the acceleration and displacement response spectra for an earthquake motion. The acceleration spectrum is used to predict the earthquake intensity that will just open the joint cracks in the wall element. The displacement spectrum is used to predict the earthquake intensity that will generate wall displacements equal to the displacement at which the wall element becomes unstable. Modification factors are applied to allow for the effect of the wall element boundary conditions and to allow for amplification of the earthquake motion due to flexibility in any building structure or diaphragms that support the wall element. The methodology was principally developed using the results of inelastic dynamic analyses of computer models of face-loaded URM wall elements. Good agreement was obtained when this type of modelling was used to predict the displacement time-history of a test specimen. The analyses indicated that the earthquake intensity required to collapse a face-loaded wall element, as indicated by the computer modelling, is generally predicted conservatively by the proposed methodology.
Paper P22: [Read]
Richard Fenwick, Barry Davidson and David Lau
Lateral load tests of reinforced concrete perimeter frames with diaphragms have shown that the addition of a floor slab (diaphragm) can have a major influence on structural performance. At the University of Auckland three moment resisting frames were tested. Two of these frames were tested without a floor slab being attached to the beams, while the remaining frame was tested with the addition of a typical floor slab containing prestressed units. The tests showed that the addition of the floor slab increased the strength of the beams appreciably and as a result the lateral strength of the frame was increased by close to 150 percent. Clearly a strength increase of this order of magnitude is of major concern in seismic design in cases where it is essential to avoid the premature formation of a column sway mechanism. The test results presented together with an analytical study show the origins of this strength increase. Understanding these mechanisms is a first step in establishing a design method for assessing over-strength values in perimeter frames, which contain floors with prestressed units.
Paper P23: [Read]
Cameron MacPherson, John Mander and Des Bull
Recent earthquake engineering research has raised concerns of the seismic performance of precast prestressed concrete hollow-core floor systems. Experimental research showed that with simple detailing enhancements, significant improvement in the seismic performance of hollow-core floor systems can be expected. The present experimental research aims at validating several new detailing enhancements. Based on previous research findings, the present super-assemblage experiment included the following details: (i) a reinforced connection that rigidly ties the floor into the supporting beam, (ii) an articulated topping slab portion cast onto a timber infill solution that runs parallel to the hollow-core units and edge beams; (iii) specially detailed supporting beam plastic hinge zones reducing potential damage to the hollow-core units; (iv) Grade 500E reinforcing steel used in the main frame elements; and (v) mild steel deformed bars in the concrete topping in lieu of the customary welded wire mesh. The full-scale structure was cyclically tested in both the longitudinal and transverse directions to inter-storey drifts of ±5%. Observations show extremely positive results with minor damage incurred by the hollow-core flooring and the overall performance dictated by the performance of the moment resisting frame. Recommendations for the forthcoming revision of the New Zealand Concrete Standard, NZS 3101, are also made.
Paper P24: [Read]